According to known designs, automatic transmissions are equipped with rotating multiple wet disc clutches and/or multiple disc brakes with a stationary support by the housing. These switch elements, which may also be used as start-up elements, are arranged or designed such that they do not dip into the oil sump, in order to avoid oil foaming and increased drag torque.
As is known, a clutch is an engageable or switchable element which selectively links two rotating shafts. A brake is an engageable or switchable element which selectively applies a force between a moving (rotating) member and a fixed member. In each case, heat may be produced as the element engages. In typical automotive automatic transmission designs, the clutch or brake is provided as a set of parallel friction plates which engage a nearby surface under influence of a selectively applied hydraulic force.
According to known designs, a separate cooling fluid supply line is provided to cool the switch elements or start-up elements. With rotating clutches, the cooling fluid is most often supplied from the inside, in order to utilize the effect of the centrifugal force for the delivery of the cooling fluid through the pad grooves of the friction pads. With closed multiple disk brakes (both actuated and synchronous), the ability of the centrifugal force to deliver cooling fluid from the inside, through the pad grooves, is no longer available; however, this is not necessary for multiple disc brakes used as switch elements, because the otherwise available cooling is sufficient for dissipating the switching energy.
DE 198 00 490 C2 describes a device for cooling two multiple disc clutches arranged on a gear shaft, with the two multiple disc clutches being arranged radially one above the other, where the supply of the fluid stream is carried out through the gear shaft, and where the fluid stream is diverted to the active clutch by means of deflectors. One disadvantage of this configuration is that a certain arrangement of the clutches is required, which may be disadvantageous, for example due to installation space constraints.
Wet multiple disc brakes are also used as start-up elements, for example as start-up brakes for the reverse gear. To remove the heat that arises due to the friction energy from the start-up brakes, these brakes are completely flooded and the fluid is exchanged. Filling the entire brake causes drag torque that affects other components as well. Furthermore, a defined fluid flow direction, which might ensure that the hot fluid is definitely exchanged and removed after heat absorption, cannot be guaranteed. Therefore, a localized thermal overload of the cooling fluid may occur, potentially resulting in poor operation or premature component failure. Such overload conditions are particularly likely, for example, if the brake is closed before the thermal load of the start-up process is dissipated, or the transmission is operated with a low slippage.
In addition, due to localized heating at a brake or clutch, the friction behavior, that is, the friction coefficient profile, may change in such an adverse manner that friction vibrations are induced in the power train, which constitutes a significant loss in comfort. In case of extreme thermal overload, the fluid filling the transmission may be stressed such that it no longer provides long term protection, and therefore, negatively impacts the cost to the user.